1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a multi-function printer, a printer, or the like that performs printing by previously forming images on image carriers and then transferring the formed images onto paper.
2. Description of Related Art
Typically, some of image forming apparatuses such as copiers, multi-function printers, and printers previously form an image (for example, toner image) on an image carrier. In such image forming apparatuses, for the purpose of synchronizing image delivery to a transfer part performing image transfer with paper arrival, a registration roller is provided. The registration roller timely enters paper into the transfer part, whereby the image is accurately and appropriately transferred onto the paper from a transfer start position of the paper. However, due to factors such as a shift of a paper loading position at a paper storing portion, a change in a conveyance distance as a result of switching a paper cassette during continuous printing, slipping at a paper feed roller performing paper feeding and/or at a conveyance roller performing paper conveyance, paper arrival at the transfer part and the registration roller may be delayed.
With the delay in the paper arrival, the image is transferred onto the paper in a displaced manner, for example, the transfer onto the paper starts from the middle of the image. As a result, there arises a case where the image is cut on the middle of the page or a case where a problem with image quality arises. Such cases require reprinting.
Thus, known is an image forming apparatus including: an image carrier; an image formation means adapted to form an image on the image carrier; a conveyance means adapted to convey recording paper; a conveyance control means adapted to stop the conveyance means to temporarily stop the recording paper and redriving the conveyance means at timing at which the image formed on the image carrier is transferred at a predetermined position of the recording paper; a timer means adapted to measure time from when the conveyance of the recording paper is stopped by the conveyance control means to when the conveyance is started again; a calculation means adapted to compare the time measured by the timer means with reference time and then based on a time difference therebetween, calculate lag time; and an adjustment means adapted to, for recording paper on which image transfer is performed following the aforementioned recording paper, cancel the lag time calculated by the calculation means and then control the conveyance means in a manner such that a period during which the recording paper is stopped is brought closer to the reference time. With this configuration, in a case where a delay in paper feed occurs due to, for example, slipping at a roller for conveying the recording paper, the adjustment means brings start of the recording paper conveyance forward by time corresponds to the lag time.
Typically, the registration roller is provided upstream of the transfer part in a paper conveyance direction. The paper is temporarily stopped by the registration roller. The temporary stopping of the paper is done for the purpose of the timely entrance into the transfer part, correcting paper skew, etc. To correct the skew, the conveyance is continued on a paper back end side to bend the paper. Then utilizing elastic properties of the bent paper, a paper leading end is placed along the registration roller to correct the skew.
On the other hand, with the delay in the paper arrival at the transfer part and the registration roller, the image cannot be transferred from a planned transfer start position of the paper and thus displaced. The more the amount of its displacement is, the more likely reprinting is to be required. Moreover, with the delay in the paper arrival, the image may arrive at the transfer part earlier, with part thereof transferred onto a component inside the transfer part, which may stain later-arriving paper. Thus, typically, a period during which the paper is stopped at the registration roller is reduced to make up for the delay in the paper arrival.
However, even when the period during which the paper is stopped at the registration roller is reduced to zero, the earlier the image arrives at the transfer part, the greater the delay in the paper arrival at the registration roller may be. That is, there is a problem of limitations on coping with the delay in the paper arrival by reducing the period during which the paper is stopped at the registration roller. In such a case, in order to prevent waste of paper, toner adhesion to the transfer part, etc., a temporarily-formed image is disposed and the same image is formed again. However, the image disposal presents a problem that the already formed image is wasted and also longer time is required for printing.
The well-known image forming apparatus described above, in a case where the delay in the paper arrival at the registration roller and the transfer part occurs constantly, is very effective in terms of resolving the delay. However, a delay in the paper arrival to a degree over a time point (limit) that permits transfer without any displacement from the transfer start position of the paper may require the image disposal. Therefore, there are some cases where the problem of limitation on coping with the delay in the paper arrival by reducing the period during which the paper is stopped at the registration roller cannot be coped with.